1. Field of the Invention
The present invention relates to an ultrasound imaging apparatus that, in a contrast echo method using an ultrasound contrast agent, presents, as diagnosis information, relative time information regarding the inflow of the ultrasound contrast agent in a region where the inflow of the ultrasound contrast agent is comparatively rapid and the other region. Further, the present invention relates to a medical image processing apparatus, a display apparatus, and a display method.
2. Description of the Related Art
An ultrasound diagnosis makes it possible to obtain a real time display of beats of the heart and movements of a fetus by a simple operation of applying an ultrasound probe to a body surface. Since an ultrasound diagnosis is highly safe, it is possible to repeatedly execute tests. Further, a system for an ultrasound diagnosis is smaller in scale than other diagnosis apparatuses, such as an X-ray diagnostic device, an X-ray Computed Tomography (CT) apparatus, and a Magnetic Resonance Imaging (MRI) apparatus. Therefore, it is possible to move an ultrasound imaging apparatus to a patient's bedside to easily and conveniently execute a test at the bedside. There are various ultrasound imaging apparatuses equipped with different types of functions, and compact-sized ultrasound imaging apparatuses that can be carried with one hand have been developed. Besides, since an ultrasound diagnosis does not involve the effects of exposure as seen in an X-ray diagnosis and other types of diagnoses, it is possible to use in obstetrics, home medical care, etc.
Ultrasound contrast agents that are administered intravenously have been commercialized, and the “contrast echo method” has become common in an ultrasound diagnosis. The object of this method is to enhance blood-flow signals by injecting an ultrasound contrast agent from a vein when executing a test of the heart, the liver, etc., and to assess the dynamic state of the blood flow. For most kinds of ultrasound contrast agents, micro bubbles function as reflection sources. For example, a second-generation ultrasound contrast agent called “Sonazoid” is composed of micro bubbles containing perfluorobutane gas with phospholipid as shell. Use of this ultrasound contrast agent allows a stable observation of the circulating current of the ultrasound contrast agent with transmission ultrasound waves of low and medium sound pressures.
When imaging a diagnosis site (for example, a liver cancer) after administering an ultrasound contrast agent to a subject, it is possible to observe the increase and decrease of the signal intensity during the inflow and outflow of the ultrasound contrast agent that is circulated with the blood flow. There have been studies indicating that differences in these moment-to-moment changes in signal intensity enable a differential diagnosis of benignancy or malignancy of a mass lesion and a diagnosis of a diffuse disease, etc.
In contrast to mere morphological information, moment-to-moment changes in signal intensity as described above typically usually need to be recorded or interpreted as motion image data. In general, a longer period of time is required for interpretation.
Therefore, a method of mapping inflow time information of an ultrasound contrast agent to be observed in motion images onto a single still image has been proposed (for example, refer to Japanese Patent Publication No. 2714329, Japanese Patent Publication No. 3495710, and Japanese Unexamined Patent Application Publication No. 2004-321688). Such a method represents, with different hues, a difference in times when signals of an ultrasound contrast agent form a peak and allows grasp of the inflow time at each site in the cross sectional surface in a single view.
However, the mapping of the inflow times of the contrast agent according to the conventional techniques involves the following problems.
The first problem is that it is difficult to set a reference time (zero point) of the moment-to-moment change. For example, in the method described in the abovementioned Japanese Patent Publication No. 2714329, a 2D image is drawn with different colors by comparing previously obtained signal intensities and newly obtained signal intensities mainly in the power Doppler imaging. When the ultrasound probe is moved three-dimensionally, it looks as if the previously obtained signals exist in the background because of the color changes due to the moment-to-moment changes. On the other hand, when the ultrasound probe is fixed, it is possible to represent, with different hues, the moment-to-moment changes of the signals due to the inflows of the ultrasound contrast agent. However, the abovementioned color changes are irrelevant to the time of administration of the ultrasound contrast agent, and it is therefore difficult to recognize, only based on the colors, absolute or relative time information, such as arrival times of the ultrasound contrast agent. In many implemented examples, a time to start injection of an ultrasound contrast agent from the vein is often set as the reference time. However, in this configuration, a difference in transmission time as the ultrasound contrast agent passes from the veins through the heart, the lungs, the heart and the arteries to an organ to be observed is also taken into consideration. Therefore, use of the value of the measurement result is not preferable as moment-to-moment changes of the organ being observed.
The second problem involves minor variation included in the measurement results. In the method described in the abovementioned Japanese Patent Publication No. 3495710, by applying different colors to two or more blood vessels having different peak values of the signals of the ultrasound contrast agent, for example, the artery and the portal vein can be identified and displayed. As in the method described in the abovementioned Japanese Patent Publication No. 2714329, this method may be applicable to relatively thick vascular systems.
However, in the actual contrast echo method, it is necessary to compare the inflow time of the ultrasound contrast agent into the entire tumor and the inflow time of the ultrasound contrast agent into the hepatic parenchyma (healthy part) around the tumor and other regions.
For example, when mapping the arrival times of the ultrasound contrast agent to the hepatic parenchyma that appears in an image, it is anticipated that the coloring is a mosaic pattern because the arrival times of the ultrasound contrast agent are not the same in all regions of the hepatic parenchyma. The mean value of these values or the degree of variation (dispersion) may be diagnostically significant. For such a diagnosis of a micro blood flow, it is difficult to favorably present diagnosis information by the method described in Japanese Patent Publication No. 3495710.
Further, according to the method described in the abovementioned Japanese Unexamined Patent Application Publication No. 2004-321688, it is possible to present the arrival time of the ultrasound contrast agent by executing more detailed operations based on a theoretical model of the circulating current of a micro blood flow.
However, it is difficult to solve the abovementioned first and second problems by the method described in Japanese Unexamined Patent Application Publication No. 2004-321688.